1. Field of the Invention
This invention relates to a lithography process, and more particularly relates to a method of measuring the numerical aperture (NA) of an exposure machine, a control wafer used in the method, a photomask for fabricating the control wafer, and a method of monitoring the NA of an exposure machine based on the above measuring method.
2. Description of Related Art
In a patterning process, the NA of the exposure machine is important since it is directly related to the resolution (R=k1λ/NA) and the depth of focus (DOF=k2λ/NA2) correlated to the accuracy of the pattern transfer. Therefore, all the exposure machines used in the same IC fabrication process should have the same NA, so as to uniform the properties of the products.
However, since the NAs of the exposure machines certainly have more or less differences, the NAs are generally compensated in use to reduce the difference as far as possible. Before the compensation, the NAs of the exposure machines are respectively measured. In a conventional method, the NA of an exposure machine is measured by a TMAP instrument manufactured by LITEL Company. However, such NA measuring instrument is very expensive and therefore not popular.